(a) Technical Field
The present disclosure relates to a diode-pumped laser apparatus. More particularly, it relates to a femtosecond laser apparatus using a laser diode optical pumping module, which can provide stable mode locking and can improve power stability and beam stability in a ultrafast laser.
(b) Background Art
In general, an ultrafast laser pulse has superior properties such as high peak power, a large spectral width, etc., as well as a short pulse time width.
The ultrafast laser system, due to such properties thereof, has been used for ultra-precision micromachining of various materials, non-linear optics, and bio, chemical, physical, and medical applications, and so forth.
For example, because of minimizing thermal diffusion in a processing region and causing no residual damage to the periphery, an ultrafast laser pulse of a femtosecond region can process a material which is too hard to be processed mechanically. And a non-linear optical effect which is multi-photon absorption based on high peak power, the ultrafast laser pulse can process a structure of various nano scales even for a transparent material such as glass, polymer, etc.
Laser stability indicates whether elements are maintained constant over time, in which the elements include power applied to an object to which a laser is to be applied, a position of a beam to be applied to the object, spatial traveling of the beam, a distribution pattern of the beam, and so forth.
When a laser has to be precisely applied to a target from a remote place, for example, like in laser processing, laser stability is very important.
In particular, for an ultrafast laser such as a femtosecond laser, laser stability is even more important in ultra-precision laser processing having nanometer-level precision.
Beam stability indicating whether or not the beam travels stably at predetermined position and angle in spatial terms is closely related to applications such as laser processing.
The characteristics of beam stability may be described with beam positional stability associated with a positional change in the form of a beam on an object and beam angular stability indicating an angular change when the beam is focused on the object.
Beam stability is typically called “beam pointing stability”.
Factors affecting laser stability may include internal and external factors such as physical vibration, mechanical deformation, a change in thermal distribution, instability of a resonator, air flow, etc.
An ultrafast laser is especially sensitive to those internal and external factors, and thus, various efforts have been made to improve the stability of the laser.
In the ultrafast laser represented by a femtosecond laser, mode-locking is a method to obtain ultrashort pulses from a femtosecond oscillator. Because mode-locking is very sensitive to a minute change in an optical path, the oscillator is also very sensitive to mechanical deformation of optical mounts caused by a change in the ambient temperature, such that the output characteristics of the oscillator may sensitively change accordingly.
For this reason, most femtosecond lasers, for stable operation, are installed and operated in clean rooms having constant-temperature facilities for stably maintaining the ambient temperature within ±0.5° C.
However, a local temperature change inevitably occurs around optical mounts such as an optical pumping-related mount, a laser medium mount, etc., to which a high-power pumping light source is applied, or around a laser power device, a cooling device, etc., which emit much heat to outside due to a cooling pan thereof.
That is, the femtosecond laser is much affected in terms of its output characteristics by a local temperature change around optical parts included therein as well as a change in the ambient temperature around a space in which the laser is installed.
Therefore, it is very important that a femtosecond laser system is mechanically configured to be sensitive as less as possible to the temperature.
A conventional representative femtosecond laser is a laser which uses Ti-Sapphire as a medium.
The Ti-Sapphire medium may generate a very short pulse of up to several femtoseconds, because of having a large emission spectral band of 100 nanometers.
To pump energy from outside, a green light source from an Nd:YVO laser pumped by a high-power laser diode is strongly focused with a magnitude of several tens through several hundreds of micrometers to a Ti-Sapphire laser crystal.
The pumping light source is structurally spaced apart from Ti-Sapphire by several meters, such that to stably operate the Ti-Sapphire laser, the stability of the output characteristics of the pumping light source is very important.
For example, with poor pointing stability of the pumping light source, a spot position of the pumping light source focused strongly with a magnitude of several tens through several hundreds of micrometers in the Ti-Sapphire laser crystal changes inconstantly, such that mode coupling between the pumping light source and the laser beam continuously changes, degrading the laser power stability.
For instance, if characteristics such as output power, output beam direction, and so forth are deteriorated, the quality of a laser-processing product using the characteristics becomes also deteriorated.
To overcome the above problem, a reflecting mirror is installed on an optical mount having mounted thereon a control device capable of finely adjusting a beam direction. Therefore, the beam direction of the pumping light source can be controlled by the reflecting mirror, thereby controlling power stability.
However, when optical parts are simultaneously controlled to optimize stability, a system becomes large in size and complex in construction and the price of the system also increases.
Therefore, in general cases, a control device is mounted only one or two optical parts.
On the other hand, if an ultrafast pulse can be obtained by applying a high-power beam output from a semiconductor laser diode directly to a laser crystal, a problem that a conventional Ti-Sapphire laser needs an expensive pumping laser can be overcome. Then, such problems of price, size, and stability, which are disadvantages of the Ti-Sapphire laser, can be more easily solved.
In this case, if a high-power laser diode, which is a pumping light source, can be positioned close to the laser crystal within several tens of centimeters or several centimeters from the laser crystal, then the stability of the ultrafast laser can be further improved.
If an amplifier part used for improving the output power of the ultrafast laser can be pumped in a continuous-wave mode, instead of a pulse mode, by using the high-power laser diode, then the laser can be further stably operated.
When a femtosecond pulse is first generated in mode locking in a femtosecond oscillator, the energy of the pulse is very low of about nano Jule (nJ), and thus the pulse is not suitable for applications such as laser processing and the like.
To increase the energy of the femtosecond pulse, the technique of chirped pulse amplification (CPA) is used.
A pulse output from a femtosecond oscillator is temporally stretched and then applied to an amplifier, thus being used as a seeding pulse.
In a laser crystal of the femtosecond amplifier, stable coupling between the seeding pulse and an amplifier/resonator mode is very important for the stability of the femtosecond amplifier.
Therefore, in a system having an amplifier in the femtosecond laser, the stability of an oscillator becomes more important.
Another way to increase the stability of the femtosecond laser is manufacturing a laser case by excavating the inside of a single aluminum block, in which other portions than an upper cover are manufactured integrally as one piece to minimize mechanical deformation of the case due to the temperature change.
Also, in an effort to reduce the temperature change, a module having a high-power diode mounted thereon or a module having a laser crystal mounted thereon is manufactured with copper having superior heat conductivity, and cooling water of a constant temperature is let flow.
To further improve the stability, a cooling water line, as well as the above-described modules, may be provided in the laser case to let the cooling water flow, thus minimizing the temperature change.
However, for example, it can be often seen that, if the laser is operated and used, and after being powered-off, the laser is powered on again for operation on the next day, then the characteristics of the laser are deteriorated.
This is because mechanical deformation occurring during power-off is not completely restored by the cooling device even after power-on.
To overcome this problem, in some common femtosecond laser system, it is recommended that the laser system such as a pumping light source, a cooling device including cooling water, etc., be operated throughout 24 hours to maintain a stable state.
However, it is not easy to maintain optical mounts of a laser resonator at a constant temperature.
To reduce a change in power of the femtosecond laser over time, a spatial change of a beam including pointing stability, or the like, mode coupling for spatially matching the pumping light source applied from outside to the beam in the laser crystal inside the resonator should be maintained stably.
If the pumping light source applied from outside is far from the laser crystal, it is not easy to stably maintain the mode coupling due to the beam stability of the pumping light source.
Therefore, to manufacture a femtosecond laser of high stability, it is desirable to apply a laser diode, as a pumping light source, directly to the laser crystal at close range.
Since the high-power pumping light source is applied from outside and the high-power laser beam is generated inside the laser resonator, the high power is delivered in the form of heat to optical mounts with which optical parts, e.g., the laser crystal, the optical mirror, etc., are engaged, and also to a laser platform and the laser.
The delivered heat causes mechanical deformation of an optical mount or the like, and the mechanical deformation minutely changes the direction of an optical part, such that the arrangement of the laser resonator is disturbed, degrading the output characteristics of the laser.
In particular, in the ultrafast laser such as the femtosecond laser, a mode locking phenomenon is used to generate a femtosecond pulse, and since mode locking is very sensitive to deformation of the resonator, the stability of the femtosecond pulse is degraded, and finally, mode locking is not maintained any more, such that the femtosecond pulse may not be even generated any more.
For example, the form of a femtosecond laser implemented as a test is shown in an optical conceptual diagram of FIG. 1.
In FIG. 1, an LC indicates a laser crystal, M1 through M6 indicate reflecting mirrors, SAM indicates a saturable absorber mirror, DM indicates a dichroic mirror, OC indicates an output coupler, LD indicates a laser diode, WP indicates a half-wave plate, CL indicates a collimating lens, and FL indicates a focusing lens, respectively.
In FIG. 1, a dotted-line block indicates an optical pumping unit of laser diode.
Conventionally, to realize the optical conceptual diagram in this form, optical parts like respective reflecting mirrors are independently mounted on optical mounts and are fixed to a laser platform.
More specifically, FIG. 2 shows a device which realizes the dotted-line block of FIG. 1, in which energy is optically provided from outside of the laser resonator to the laser crystal inside the resonator by using the high-power laser diode.
Herein, a reference numeral 100 indicates an optical fiber, reference numerals 110a through 110f indicate respective mounts for mounting the optical fiber, the half-wave plate, the collimating lens, the focusing lens, and the dichroic mirror thereon, and reference numerals 120a through 120g indicate mount blocks installed on a laser platform 130 to support the respective mounts.
When a high-power diode output beam passes through various optical parts, some portion thereof is reflected and hit on various mechanical parts or a laser case.
Then, as the high-power pumping beam is absorbed in some optical mounts, thermal distribution in the laser becomes non-uniform, resulting in local heating, such that the respective mounts are independently deformed and thus, the arrangement of the resonator of the laser becomes poor.
As such, if much deformation occurs, mode locking for generating the femtosecond laser pulse in the femtosecond laser is not maintained any more, and thus the femtosecond pulse is not generated any more and a continuous wave of a very low peak power is generated.
In this case, a mode-locking starter has to be operated to generate mode locking again, such that mode locking is generated and the femtosecond laser pulse is generated again.
If the laser, although having been mode-locked, is left powered-off for a long time, the laser is thermally cooled down and returns to the thermal equilibrium state.
However, if the laser is powered on again, thermal imbalance occurs again, causing mechanical deformation.
However, since the laser does not completely return to the previous state, mode locking is often broken.
FIG. 3 shows a change in power over time when the femtosecond laser starts operating in a state where optical mounts of an optical pumping unit are mounted independently.
FIG. 3 shows the case when the laser is powered on again after the laser, which was operated previously to stably generate the femtosecond laser pulse, was powered off to cool completely.
As can been seen, although mode locking of the femtosecond laser was stable, if the laser is powered on again, mode locking is often broken and thus a continuous wave (CW) is generated. As a result, the mode-locking starter has to be operated for mode locking of the laser.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.